Flue gas desulfurization process and apparatus for removing nitrogen oxides

ABSTRACT

An apparatus and process for removing acidic gases and NOx from flue gases produced by utility and industrial plants. The process and apparatus convert NOx, and particularly nitric oxide, to nitrogen dioxide, which is then reacted to form a valuable byproduct. The process generally entails contacting a flue gas with a scrubbing medium to absorb acidic gases from flue gas and produce an intermediate flue gas. The intermediate flue gas is then cooled to cause nitric oxide present therein to be oxidized to form nitrogen dioxide, which is then absorbed from the flue gases to produce a nitrogen dioxide-containing solution and a scrubbed flue gas. The nitrogen dioxide in the nitrogen dioxide-containing solution is then reacted with ammonium hydroxide to form ammonium nitrate as a valuable byproduct.

This application is a division of U.S. application Ser. No. 10/064,055filed on Jun. 5, 2002, now U.S. Pat. No. 6,759,022.

BACKGROUND OF INVENTION

1. Field of the Invention

This invention generally relates to gas-liquid contactors used in theremoval of acidic gases, such as from utility and industrial flue gases.More particularly, this invention is directed to a wet flue gasdesulfurization (FGD) process and apparatus that remove nitrogen oxides(NOx) from a flue gas following the removal of acidic gases such assulfur dioxide.

2. Description of the Related Art

Acidic gases, including sulfur dioxide (SO₂), hydrogen chloride (HCl)and hydrogen fluoride (HF), are known to be hazardous to theenvironment, and as a result their emission into the atmosphere isclosely regulated by clean air statutes. For the removal of acidic gasesfrom flue gases produced by utility and industrial plants, gas-liquidcontactors and absorbers, or scrubbers, are widely employed. Scrubbersgenerally employ a liquid media that is brought into intimate contactwith a flue gas to remove acidic gases by absorption. The process bywhich acidic gases are removed from flue gases in this manner isgenerally referred to as wet flue gas desulfurization (wet FGD).

The cleansing action produced by scrubbers is generally derived from thepassage of a flue gas through a tower cocurrently or countercurrently toa descending liquid medium. Calcium-based slurries, sodium-basedsolutions and ammonia-based solutions are typical alkaline scrubbingmedia used in flue gas scrubbing operations. The cleansed gases areallowed to exit the tower, typically passing through a mist eliminatorto atmosphere. The liquid medium and its absorbed gases are collected ina tank, typically at the bottom of the tower, where the absorbed gasesare reacted to form byproducts that are useful or at least not harmfulto the environment. While scrubbers utilizing calcium-based slurriesgenerally perform satisfactorily, their operation results in theproduction of large quantities of wastes or gypsum, the latter havingonly nominal commercial value. In contrast, ammonia-based scrubbingprocesses have been used in the art to produce a more valuable ammoniumsulfate fertilizer, as taught by U.S. Pat. Nos. 4,690,807 and 5,362,458,each of which are assigned to the assignee of the present invention. Inthese processes, the scrubbing solution is accumulated in a tank wherethe absorbed sulfur dioxide reacts with ammonia (NH₃) to form ammoniumsulfite ((NH₄)₂SO₃) and ammonium bisulfite (NH₄HSO₃), which are oxidizedin the presence of sufficient oxygen to form ammonium sulfate((NH₄)₂SO₄) and ammonium bisulfate (NH₄HSO₄), the latter of which reactswith ammonia to form additional ammonium sulfate. A portion of theammonium sulfate solution and/or ammonium sulfate crystals that form inthe solution can then be drawn off to yield the desired byproduct ofthis reaction.

Nitrogen oxides (NOx), which include nitric oxide (NO) and nitrogendioxide (NO₂), are also commonly found in flue gases produced by utilityand industrial plants. The presence of NOx in the effluent of a scrubberis environmentally undesirable because of links to smog and ozonedeterioration. Because nitric oxide readily oxidizes at room temperature(about 20° C. to about 25° C.) to form nitrogen dioxide, nitric oxidepresent in flue gases tends to form nitrogen dioxide which, depending onatmospheric conditions, can produce an unsightly yellow-brown plume. Theelimination of NOx from flue gases has typically focused on theavoidance of forming nitrogen dioxide. For example, nitrogen gas (N₂) isproduced in processes involving NOx dissociation by selective catalyticreduction (SCR) or selective non-catalytic reduction (SNCR). As theprimary component of air, the release of nitrogen gas into theatmosphere does not pose an environmental concern. However, an ongoingdemand of desulfurization processes is the reduction of emissions. Froman economics standpoint, it is also desirable for a desulfurizationprocess to produce valuable byproducts. Electron beam and low NOxburners have also been proposed as methods for removing Nox. Forexample, irradiation with an electron beam has been used to convert NOxto ammonium nitrate (NH₄NO₃) in the presence of ammonia, as disclosed inU.S. Pat. Nos. 5,834,722 and 6,179,968. However, disadvantages of ionbeam treatments include their relatively high cost and low efficiency.

SUMMARY OF INVENTION

The present invention provides an apparatus and process for removingacidic gases and NOx from flue gases produced by fuel combustionoperations of the type carried out in utility and industrial plants. Theprocess and apparatus of this invention intentionally convert NOx, andparticularly nitric oxide, to nitrogen dioxide, which is then reacted toform a valuable byproduct. As such, the process and apparatus of thisinvention differs from prior processes in which the object was toconvert NOx to nitrogen gas, which is then released into the atmosphere.

The process of this invention generally entails contacting a flue gaswith a scrubbing medium, preferably an ammonium sulfate-containingscrubbing solution, to absorb acidic gases from the flue gas and producean acidic gas-containing solution and an intermediate flue gas.According to the invention, after contact with the scrubbing medium, theintermediate flue gas is cooled to cause nitric oxide to be oxidized toform nitrogen dioxide, which is then absorbed from the intermediate fluegas to produce a NO₂-containing solution and a more fully scrubbed fluegas. The nitrogen dioxide in the NO₂-containing solution is then reactedwith an aqueous ammonia solution to form ammonium nitrate (NH₄NO₃) asthe valuable byproduct of the process. A flue gas scrubbing apparatussuitable for use with this invention comprises means for contacting theflue gas with the scrubbing medium to remove acidic gases and producethe intermediate flue gas, means for reducing the temperature of theintermediate flue gas to convert nitric oxide present in theintermediate flue gas to nitrogen dioxide, means for absorbing nitrogendioxide from the intermediate flue gas to produce the NO₂-containingsolution and a scrubbed flue gas, and means for reacting the nitrogendioxide in the NO₂-containing solution with aqueous ammonia solution toproduce ammonium nitrate.

According to the above, NOx present in a flue gas is intentionallyconverted to nitrogen dioxide, which is the species typically avoided inprior art processes because of environmental concerns and the visibleplume produced by nitrogen dioxide. However, the present invention usesa combination of oxidation of nitric oxide to nitrogen dioxide, followedby a controlled reaction with an aqueous ammonia solution, the result ofwhich does not reduce NOx to nitrogen gas but instead captures NOx toproduce a valuable byproduct.

Other objects and advantages of this invention will be betterappreciated from the following detailed description.

BRIEF DESCRIPTION OF DRAWINGS

The present invention will now be described, by way of example, withreference to FIG. 1, which is a schematic representation of a flue gasscrubbing apparatus configured in accordance with the present invention.

DETAILED DESCRIPTION

FIG. 1 schematically illustrates a flue gas scrubbing apparatus 10 thathas been modified in accordance with the teachings of this invention.The scrubber 10 is generally of the type that scrubs flue gases producedby the burning of fossil fuels or another process that results in theflue gas containing acidic gases, such as sulfur dioxide, hydrogenchloride and/or hydrogen fluoride, as well as particulate matter and, ofinterest to the present invention, nitrogen oxides (NOx).

The conventional components of the scrubber 10 include a contact region16 in which an alkaline contact medium, referred to as a scrubbingslurry or solution 14, is brought into contact with a flue gas thatenters the scrubber 10 through an inlet duct 12. The solution 14 isshown as being delivered with a pump 22 through a pipe 46 to the contactregion 16, where the solution 14 is dispersed with spray nozzles 20 oranother suitable delivery device. After being scrubbed by the solution14, the flue gas flows up through a demister 17, and is eventuallyreleased to atmosphere through a chimney 24 or other suitable structure.As with many existing wet flue gas desulfurization facilities, thescrubber 10 is equipped for in situ forced oxidation of the solution 14that has collected in a tank 18 below the contact region 16. In FIG. 1,an oxygen-containing gas (e.g., air) is represented as being introducedinto the tank 18 with a sparger 26 connected to a suitable source 28. Inthis manner, the reaction product of contacting the acidic gases of theflue gas with the solution 14 is oxidized, which in the presentinvention preferably yields a useful fertilizer byproduct. A particularexample is ammonia-based scrubbing processes taught by commonly-assignedU.S. Pat. Nos. 4,690,807 and 5,362,458. In these processes, acidic gasespresent in a flue gas are absorbed by an ammonium sulfate solution 14,which collects in the tank 18 where aqueous ammonia (ammonium hydroxide,NH₄OH) or another source of ammonia is introduced, such as with thesparger 26. The absorbed sulfur dioxide reacts with the ammonia to formammonium sulfite (NH₄)₂SO₃ and ammonium bisulfite (NH HSO₃), which arethen oxidized in the presence of sufficient oxygen (introduced by thesparger 26) to form precipitates of ammonium sulfate and ammoniumbisulfate (NH₄HSO₄). Ammonium bisulfate undergoes a second reaction withammonia to form additional ammonium sulfate precipitate. A portion ofthe ammonium sulfate solution 14 is then removed through a pipe 50 anddewatered with a suitable dewatering device 30 to precipitate ammoniumsulfate, which can then be sold as a valuable fertilizer. If hydrogenchloride and hydrogen fluoride were present in the flue gas, as istypically the case with flue gas produced by the combustion of coal,these acidic gases are also captured to form ammonium chloride andammonium fluoride, which can be removed in the same manner. Furtherdetails regarding the desulfurization of flue gases can be obtained inthe prior art, including the above-noted U.S. Pat. Nos. 4,690,807 and5,362,458, and therefore will not be discussed in any further detailhere.

In addition to the above, the scrubber 10 of this invention removes NOxfrom the scrubbed flue gas emerging from the scrubbing process performedin the contact region 16. The flue gas that has passed through thecontact region 16 will be referred to here as an “intermediate” fluegas, since it is not the final “scrubbed” flue gas that will be releasedto atmosphere through the chimney 24. Further processing of theintermediate flue gas occurs in additional zones provided between thedemister 17 and the chimney 24 of the scrubber 10. A first of thesezones is a heat exchanger 32 or other device capable of reducing thetemperature of the intermediate flue gas to something below theauto-oxidation temperature of nitric oxide (NO), which readily oxidizesto form NO₂ at or near room temperature. In conventional scrubbers ofthe type represented in FIG. 1, flue gases exit the contact region 16 ata temperature of typically at least 125° F. (about 50° C.), and oftenhigher, such that the nitrogen dioxide content of the flue gas isrelatively low, i.e., the NOx content of flue gases is primarily NO. Ina conventional FGD process, oxidation of nitric oxide occurs after thescrubbed flue gases are released to atmosphere. A preferred heatexchanger 32 is capable of reducing the flue gas temperature so that themajority of the nitric oxide content of the intermediate flue gas isconverted to nitrogen dioxide, e.g., the flue gas is reduced from atemperature of about 125EF or higher to something near or below roomtemperature. Various types of heat exchangers known to those skilled inthe art are believed to be suitable for use in the scrubber 10 shown inFIG. 1, and therefore the construction of the heat exchanger 32 will notbe discussed here in any detail.

After conversion of nitric oxide to nitrogen dioxide within the heatexchanger 32, the intermediate flue gas passes through an absorptionzone 34 where nitrogen dioxide is absorbed so that the scrubbed flue gasexiting the chimney 24 is essentially free of nitrogen dioxide. Nitrogendioxide is more readily absorbed in water and ammonia solutions than isnitric oxide. In a preferred embodiment, the flue gas is contacted withboth water (or a water-containing solution) and an ammonia-containingsolution, which are introduced into the absorption zone 34 asrepresented in FIG. 1. Absorption of nitrogen dioxide with water formsnitric acid (HNO₃), while absorption of nitrogen dioxide with aammonia-containing (e.g., ammonium sulfate or hydroxide) solutionproduces a blend of desirable fertilizers, as discussed below. Water isshown as being delivered from a source 38, while the ammonia-containingsolution is represented as being recycled through a pipe 40 from asolution collection tank 44 below the absorption zone 34. Theammonia-containing solution, designated with reference number 36 in FIG.1, is collected from the absorption zone 34, and therefore is an aqueousammonia solution containing absorbed nitrogen dioxide. The water and theammonia-containing solution 36 can be brought into intimate contact withthe flue gas within the absorption zone 34 in any suitable manner, suchas with spray nozzles or packed column.

The ammonia-containing solution 36 preferably has a pH of less than 7and contains ammonia slightly above the stoichiometric amount. Asuitable and convenient source of the ammonia-containing solution 36 foruse in the absorption zone 34 is the ammonium sulfate solution 14 usedin the contact region 16 to remove the acidic gases from the flue gas,such that the ammonium hydroxide present in the solution 14 is thesource of ammonia in the solution 36. A secondary benefit of using theammonium sulfate solution 14 is that the absorption zone 34 provides asecond opportunity for removing sulfur dioxide and other acidic gasesfrom the flue gas. For the purpose of delivering the solution 14 to theabsorption zone 34, the solution collection tank 44 is shown as beingcoupled by a pipe 48 to the pipe 46 connected to the pump 22. Thesolution collection tank 44 is also shown as being connected by a pipe42 through which the ammonia-containing solution 36 can be returned tothe tank 18. The final reaction of the process is the conversion of theabsorbed nitrogen dioxide in the ammonia-containing solution 36 into avaluable byproduct, ammonium nitrate (NH₄NO₃), which occurs according tothe following reaction:NO₂+NH₄OH→NH₄NO₃+H

The ammonium nitrate formed or otherwise collected in the tank 18 can bewithdrawn and dewatered in the same manner as the ammonium sulfateproduced by the absorption of sulfur dioxide.

In view of the above, the present invention can be seen as providing amethod and apparatus for capturing nitrogen oxides from a flue gas.Benefits of this invention include the substantial elimination ofundesirable NOx emissions from the scrubber 10, producing a usefulnitrogen-containing byproduct instead of releasing nitrogen gas toatmosphere, and providing a second opportunity for additional removal ofsulfur dioxide, leading to a cleaner scrubbed flue gas released to theatmosphere.

While the invention has been described in terms of a preferredembodiment, it is apparent that other forms could be adopted by oneskilled in the art. Accordingly, the scope of the invention is to belimited only by the following claims.

1. A flue gas scrubbing apparatus for removing NOx gases from a fluegas, the flue gas scrubbing apparatus comprising: means for contactingthe flue gas with a scrubbing medium so that the scrubbing mediumabsorbs acidic gases from the flue gas to produce an acidicgas-containing solution and an intermediate flue gas; means for reducingthe temperature of the intermediate flue gas to convert nitric oxide tonitrogen dioxide; means for absorbing the nitrogen dioxide from theintermediate flue gas to produce a nitrogen dioxide-containing solutionand a scrubbed flue gas; and means for reacting the nitrogen dioxide inthe nitrogen dioxide-containing solution with ammonium hydroxide toproduce ammonium nitrate.
 2. A flue gas scrubbing apparatus according toclaim 1, wherein the absorbing means comprises means for contacting theintermediate flue gas with a water-containing solution so that thewater-containing solution absorbs the nitrogen dioxide from theintermediate flue gas to form the nitrogen dioxide-containing solution.3. A flue gas scrubbing apparatus according to claim 2, wherein thereacting means comprises means for contacting the nitrogendioxide-containing solution with an ammonia-containing solutioncontaining the ammonium hydroxide.
 4. A flue gas scrubbing apparatusaccording to claim 2, wherein the scrubbing medium is an ammoniumsulfate-containing solution containing ammonium hydroxide, and whereinthe reacting means comprises means for contacting the nitrogendioxide-containing solution with the ammonium sulfate-containingsolution.
 5. A flue gas scrubbing apparatus according to claim 1,wherein the absorbing means comprises means for contacting theintermediate flue gas with an ammonia-containing solution so that theammonia-containing solution absorbs the nitrogen dioxide from theintermediate flue gas to produce the nitrogen dioxide-containingsolution.
 6. A flue gas scrubbing apparatus according to claim 1,wherein the scrubbing medium is an ammonium sulfate-containing solution.7. A flue gas scrubbing apparatus according to claim 6, wherein theabsorbing means comprises means for contacting the intermediate flue gaswith the ammonium sulfate-containing solution so that the ammoniumsulfate-containing solution absorbs the nitrogen dioxide from theintermediate flue gas to produce the nitrogen dioxide-containingsolution.
 8. A flue gas scrubbing apparatus according to claim 1,further comprising a vessel in which the acidic gas-containing solutionis accumulated, and means for introducing ammonia and oxygen into thevessel to react the acidic gases in the acidic gas-containing solutionto produce ammonium sulfate.
 9. A flue gas scrubbing apparatus accordingto claim 8, wherein the nitrogen dioxide-containing solution isaccumulated in the vessel, where the ammonia introduced into the vesselforms the ammonium hydroxide that reacts with the nitrogen dioxide inthe nitrogen dioxide-containing solution to produce ammonium nitrate.10. A flue gas scrubbing apparatus according to claim 1, furthercomprising means for releasing the scrubbed flue gas to atmosphere. 11.A flue gas scrubbing apparatus for removing nitric oxide gas and acidicgases from a flue gas, the flue gas scrubbing apparatus comprising:means for contacting the flue gas with an ammonium sulfate-containingscrubbing solution so that the scrubbing solution absorbs acidic gasesfrom the flue gas to produce an acidic gas-containing solution and anintermediate flue gas; a vessel containing the ammoniumsulfate-containing scrubbing solution and in which the acidicgas-containing solution is accumulated; means for reducing thetemperature of the intermediate flue gas to convert nitric oxide withinthe intermediate flue gas to nitrogen dioxide; means for contacting theintermediate flue gas with a water-containing scrubbing solution so thatthe water-containing scrubbing solution absorbs the nitrogen dioxidefrom the intermediate flue gas to produce a nitrogen dioxide-containingsolution and a scrubbed flue gas; means for releasing the scrubbed fluegas to atmosphere; means for accumulating the nitrogendioxide-containing solution in the vessel containing the ammoniumsulfate-containing scrubbing solution; and means for introducing ammoniaand oxygen into the vessel to react the acidic gases in the acidicgas-containing solution to produce ammonium sulfate and to react thenitrogen dioxide in the nitrogen dioxide-containing solution to produceammonium nitrate.
 12. A flue gas scrubbing apparatus according to claim11, further comprising means for contacting the intermediate flue gaswith the ammonium sulfate-containing scrubbing solution, such that thenitrogen dioxide-containing solution also contains ammonium sulfate. 13.A flue gas scrubbing apparatus according to claim 11, wherein thewater-containing scrubbing solution used in the step of contacting theintermediate flue gas comprises the ammonium sulfate-containingscrubbing solution, such that the nitrogen dioxide-containing solutioncontains ammonium sulfate and water.
 14. A flue gas scrubbing apparatusaccording to claim 11, further comprising means for withdrawing aportion of the ammonium sulfate-containing scrubbing solution fromvessel, and means for dewatering the portion of the ammoniumsulfate-containing scrubbing solution to precipitate ammonium sulfateand ammonium nitrate.